1. Field of the Invention
The present invention relates to a new and improved optical cuvette. The cuvette of the present invention allows the making of improved spectrophotometric measurements in turbid media such as whole blood.
2. BACKGROUND OF THE INVENTION
The present invention relates to a disposable optical cuvette #or making spectrophotometric measurements in turbid media such as whole blood and a method of constructing the cuvette. Spectrophotometric measurements on certain compounds that absorb light strongly or those on turbid samples that scatter light appreciably, require a relatively short optical pathlength. The optical pathlength from one cuvette to the next must be consistent for spectrophotometric measurements to be accurate. Therefore, the method of construction of cuvettes must be uniformly controlled.
Numerous approaches have been taken to resolve the problem of uniformly constructing cuvettes with a short optical pathlengths. U.S. Pat. 4,753,776 to Hillman discloses a cuvette for separating plasma from blood by filtration to resolve the light scattering problem when measuring whole blood. The reason Hillman separates whole blood into plasma and red blood cells is that red blood cells scatter and absorb light and could adversely affect a measurement of either reflected or transmitted light of a diagnostic test relying on either of these measurement techniques.
The optical distance through the sample of interest, i.e., width dimension, in an optical chamber is the optical pathlength, and the amount of light absorbed by the sample is directly proportional to the optical pathlength. Therefore, the optical pathlength varies from one cuvette to another, varying degrees of light absorbance and light scattering will induce error in spectrophotometric measurements.
Attempts to control light scattering by utilizing a defined volume chamber is disclosed in U.S. Pat. Nos. 4,963,498 and 4,756,884 to Hillman, et al. Hillman, et al., rely upon injection molding of a multi-layered cuvette to define the optical chamber. The multi-layered cuvette is then ultrasonically welded together. Assembling Hillmanz's cuvette is a multi-step operation involving repeated ultrasonic bonding of each surface to its adjoining surface. This particular construction technique limits the lower limit of the optical pathlength due to the shrinkage found in the component parts of a multi-layer cuvette after injection molding. Hillman, et al., disclose an optical pathlength to be equal to or greater than 180 micrometers.
Another approach to control the optical pathlength is disclosed by U.S. Pat. No. 4,585,561 to Thornton, et al. This patent discloses a cuvette with two chambers. One chamber holds the sample of interest, i.e., whole blood and a second is an over-flow chamber. The first chamber that holds the sample of interest has an optical pathlength of 0.2 cm. That lower optical pathlength is achieved by vacuum forming a portion of the optical chamber. The over-flow chamber is an attempt to remove air bubbles from the sample. Those air bubbles will cause inconsistent readings when making spectrophotometric measurements on whole blood.
The general type and construction of cuvettes disclosed in '498 and '561 are also disclosed in U.S. Pat. Nos. 3,690,836, 4,761,381 and 4,762,798. All the patents have as one of their objectives the measurement of turbid media by optical means, all with varying degrees of success. Whole blood both scatters and absorbs light making optical measurements difficult at best. Past methods have unsuccessfully attempted to remedy this problem by injection molding, and vacuum forming sample volume cavities, thereby controlling the optical pathlength.
The problem of producing a consistent, very low optical pathlength through turbid media, such as whole blood, has not been solved.